Artificial light may be generated in many ways, including, electroluminescent illumination (e.g., light-emitting diodes), incandescent illumination (e.g., conventional incandescent lamps and thermal light sources) and gas discharge illumination (e.g., fluorescent lamps, xenon lamps, and hollow cathode lamps). Light may also be emitted via direct chemical radiation discharge of a photoluminescent (e.g., chemoluminescence, fluorescence, or phosphorescence).
A light-emitting diode (LED) is essentially a p-n junction semiconductor diode that emits a monochromatic light when operated under forward bias. In the diode, current flows easily from the p-side to the n-side but not in the reverse direction. When two complementary charge carriers (i.e., an electron and a hole) collide, the electron-hole pair experiences a transition to a lower energy level and emits a photon. The wavelength of the light emitted depends on the difference between the two energy levels, which in turn depends on the band-gap energy of the materials forming the p-n junction.
LEDs are used in various applications, including, traffic signal lamps, large-sized full-color outdoor displays, various lamps for automobiles, solid-state lighting devices, flat panel displays, and the like. The basic structure of an LED consists of the light-emitting semiconductor material, also known as the bare die, and numerous additional components designed for improving the performance of the LED. These components may include a light-reflecting cup mounted below the bare die, a transparent encapsulation (typically silicone) surrounding and protecting the bare die and the light reflecting cup, and bonders for supplying the electrical current to the bare die. The bare die and the additional components are efficiently packed in a LED package.
The LED has won remarkable attention as a next-generation small-sized light-emitting source. The LED has heretofore had advantages such as a small size, high resistance and long life, but has mainly been used as indicator illumination for various measuring meters or a confirmation lamp in a control state because of restrictions on a light-emitting efficiency and light-emitting output. However, in recent years, the light-emitting efficiency has rapidly been improved, and may soon exceed that of a high-pressure mercury lamp or a fluorescent lamp of a discharge type. Due to the appearance of the high-efficiency high-luminance LED, a high-output light-emitting source using the LED has rapidly assumed a practicability. In recent years, a blue LED has been brought into practical use, complementing conventional red and green LEDs, and this has also accelerated the application of the LED.
The high-efficiency high-luminance LED has been considered as a promising small-sized light-emitting source of an illuminating unit having a light-condensing capability. The LED has characteristics superior to those of other light-emitting sources, such as life, durability, lighting speed, and lighting driving circuit. Furthermore, the availability of the three primary colors has enlarged an application range of a full-color image displays.
LEDs also represent an attractive alternative light source for general lighting applications. Solid-state LEDs consume less power than incandescent light bulbs and may have lifetimes in excess of 100,000 hours. Besides producing little heat and being energy-efficient, LEDs are smaller and less vulnerable to breakage or damage due to shock or vibration than incandescent bulbs. LED characteristics generally also do not change significantly with age. Moreover, LEDs can be used to create luminaires having novel form factors incompatible with most incandescent bulbs. More widespread luminaire design efforts not constrained by traditional incandescent form limitations will increase adoption of LED-based lighting and reap the energy savings associated therewith.
Luminescence is a phenomenon in which energy is absorbed by a substance, commonly called a luminescent, and emitted in the form of light. The absorbed energy may be in a form of light (i.e., photons), electrical field, or colliding particles (e.g., electrons). The wavelength of the emitted light differs from the characteristic wavelength of the absorbed energy (the characteristic wavelength equals hc/E, where h is the Plank's constant, c is the speed of light and E is the energy absorbed by the luminescent). Luminescence may be classified by excitation mechanism as well as by emission mechanism. Examples of such classifications include photoluminescence, electroluminescence, fluorescence, and phosphorescence. Similarly, luminescent materials may be classified into photoluminescent materials, electroluminescent materials, fluorescent materials, and phosphorescent materials, respectively.
A photoluminescent is a material which absorbs energy in the form of light, an electroluminescent is a material which absorbs energy is in the form of electrical field, a fluorescent material is a material which emits light upon return to the base state from a singlet excitation, and a phosphorescent material is a material which emits light upon return to the base state from a triplet excitation.
In fluorescent materials, or fluorophores, the electron de-excitation occurs almost spontaneously, and the emission ceases when the source which provides the exciting energy to the fluorophore is removed.
In phosphor materials, or phosphors, the excitation state involves a change of spin state which decays only slowly. In phosphorescence, light emitted by an atom or molecule persists after the exciting source is removed.
Luminescent materials are selected according to their absorption and emission characteristics and are widely used in cathode ray tubes, fluorescent lamps, X-ray screens, neutron detectors, particle scintillators, ultraviolet (UV) lamps, flat-panel displays, and the like. Luminescent materials, particularly phosphors, may also be used for altering the color of LEDs. Since blue light has a short wavelength (compared, e.g., to green or red light), and since the light emitted by the phosphor generally has a longer wavelength than the absorbed light, blue light generated by a blue LED may be readily converted to produce visible light having a longer wavelength. For example, a blue LED coated by a suitable yellow phosphor can emit white light. The phosphor absorbs the light from the blue LED and emits in a broad spectrum, with a peak in the yellow region. The photons emitted by the phosphor and the non-absorbed photons emitted of the LED are perceived together by the human eye as white light. The first commercially available phosphor based white LED was produced by Nichia Co. and consisted of a gallium indium nitride (InGaN) blue LED coated with a yellow phosphor.
In order to get sufficient brightness, a high-intensity LED is needed to excite the phosphor to emit the desired color. As commonly known, white light is composed of various colors of the whole range of visible electromagnetic spectrum. In the case of LEDs, only the appropriate mixture of complementary monochromatic colors can cast white light. This is typically achieved by having at least two complementary light sources in the proper power ratio. A “fuller” light (similar to sunlight) may be achieved by adding more colors. Phosphors are usually made of zinc sulfide or yttrium oxides doped with certain transition metals (Ag, Mn, Zn, etc.) or rare earth metals (Ce, Eu, Tb, etc.) to obtain the desired colors.
In a similar mechanism, white LEDs may also be manufactured using a fluorescent semiconductor material instead of a phosphor. The fluorescent semiconductor material serves as a secondary emitting layer, which absorbs the light created by the light-emitting semiconductor and reemits yellow light. The fluorescent semiconductor material, typically an aluminum gallium indium phosphide (AlGaInP), is bonded to the primary source wafer.
Another type of light-emitting device is an organic light emitting diode (OLED) which makes use of thin organic films. An OLED device typically includes an anode layer, a cathode layer, and an organic light-emitting layer containing an organic compound that provides luminescence when an electric field is applied. OLED devices are generally (but not always) intended to emit light through at least one of the electrodes, and may thus include one or more transparent electrodes.
Combinations of LEDs, OLEDs, and luminescence are widely used in the field of electronic display devices. Many efforts have been made to research and develop various types of such devices. Electronic display devices may be categorized into active-display devices and passive-display devices. The active-display devices include the cathode ray tube (CRT), the plasma display panel (PDP), and the electroluminescent display (ELD). The passive-display devices include a liquid crystal display (LCD), the electrochemical display (ECD), and the electrophoretic image display (EPID).
In active-display devices, each pixel radiates light independently. Passive-display devices, on the other hand, do not produce light within the pixel and the pixel is only able to block light. In LCD devices, for example, an electric field is applied to liquid-crystal molecules, and an alignment of the liquid-crystal molecule is changed depending on the electric field, to thereby change optical properties of the liquid crystal, such as double refraction, optical rotatory power, dichroism, light scattering, etc. Since LCDs are passive, they display images by reflecting external light transmitted through an LCD panel or by using the light emitted from a light source, e.g., a backlight assembly, disposed below the LCD panel.
An LCD includes a LCD panel and backlight assembly. The LCD panel includes an arrangement of pixels, which are typically formed of thin-film transistors fabricated on a transparent substrate coated by a liquid-crystal film. The pixels include three color filters, each of which transmits one-third of the light produced by each pixel. Thus, each LCD pixel is composed of three sub-pixels. The thin-film transistors are addressed by gate lines to perform display operation by way of the signals applied thereto through display signal lines. The signals charge the liquid-crystal film in the vicinity of the respective thin-film transistors to effect a local change in optical properties of the liquid crystal film.
A typical LED backlight assembly includes a source of white light, a light-guiding plate for guiding the light toward the LCD panel, a reflector disposed under the light-guiding plate to reflect the light leaked from the light-guiding plate back toward the light-guiding plate, and optical sheets for enhancing brightness of the light exiting from the light-guiding plate. Backlight assemblies are designed to achieve many goals, including high brightness, large-area coverage, uniform luminance throughout the illuminated area, controlled viewing angle, small thickness, low weight, low power consumption, and low cost.
In operation, a backlight assembly produces white illumination directed toward the LCD pixels. The optical properties of the liquid-crystal film are locally modulated by the thin-film transistors to create a light-intensity modulation across the area of the display. The color filters colorize the intensity-modulated light emitted by the pixels to produce a color output. By selective opacity modulation of neighboring pixels of the three-color components, selected intensities of the three component colors are blended together to selectively control color light output. Selective blending of three primary colors, i.e., red, green, and blue (RGB), generally produces a full range of colors suitable for color display purposes.
LCD devices are currently employed in many applications (cellular phones, personal acceptance devices, desktop monitors, portable computers, television displays, etc.), and there is a growing need to devise high-quality backlight assemblies for improving the image quality in these applications.
Since the light from the backlight must pass through the color filters, it therefore must include a wavelength at which the respective filter is transparent. However, the use of white LEDs composed of blue LEDs coated by yellow phosphors is often not efficient for backlighting because, although such dichromatic light appears as white light to the human eye, it cannot efficiently pass through RGB color filters. Another potential approach is the use of red, green, and blue LEDs that match the central wavelength of each color filter. This approach significantly complicates the manufacturing process because the red, green, and blue LEDs must be accurately aligned in a multichip approach. An additional approach is to generate white light using a UV LED and three different phosphors, each emitting light at a different wavelength (e.g., red, green and blue). The efficiency of this configuration, however, is very low because a high amount of heat is released due to the Stokes shift.
Furthermore, traditional LEDs utilizing phosphors suffer from low conversion efficiency because (i) up to 60% of the emitted light (both unconverted and converted by the phosphor) is reflected back into the chip and lost, (ii) the phosphor material is positioned proximate to the LED and is heated thereby, reducing its conversion efficiency, and (iii) light absorbed by the LED creates deleterious heating which reduces the LED efficiency. Current phosphor-converted LEDs have conversion efficiencies of only about 50% to 55% due to these issues.
Presently known LED-based backlight devices are limited by the size, price and performance of the LEDs. To date, the performance of the LED is controlled by its transparent encapsulation (which provides the necessary light scattering), the phosphor or fluorescent semiconductor material which is responsible for color conversion, and the lead frame which allows for heat evacuation, all of which significantly increase the size and cost of the LED. Since the performance, cost, and size of the LED are conflicting features, some compromises are inevitable.
There is thus a widely recognized need for, and it would be highly advantageous to have, a diode-based illumination apparatus devoid of the above limitations.